Prior art panoramic dental X-ray machines are well known. Some provide a continuous image of the dental arch area and commonly employ an X-ray source and X-ray film both optically aligned with each other and supported on a rotatable carrying arm which orbits a patient situated in the path of the X-ray beams. The patient may remain stationary or be transported in a patient chair in accordance with various type drive mechanisms in order to simulate the generally elliptical shape of the human dental arch. The continuous image radiograph provides the dentist with a panoramic view of the teeth and associated structures and is therefore a useful diagnostic aid in many phases of dental practice.
Various other prior art apparatus provide a discontinuous, or split image panoramic radiograph which possesses certain advantages. Here, the dentist is presented with additional interpretive information since two distinctly different views of the incisors, or centrals area are provided. Additionally, overlying spinal shadows which would be cast over the central-bicuspid region are eliminated since X-rays are not generated when the spine is aligned with the X-ray source and film.
Regardless of the type radiographic image to be obtained, i.e., continuous or discontinuous, compensation is usually made for the fact that the curvature of the desired area of focus is generally not a true circle or ellipse. Thus, the rate of film travel must be varied in accordance with the rate of travel of the X-ray source about the patient's head in order that the radiological projections occupy a distance on the film equal to the linear distance of a curved structure being X-rayed, such as a typical dental arch.
In U.S. Pat. No. 2,798,958, apparatus is disclosed for varying the rate of film travel relative to the rate of travel of the X-ray source. The X-ray source and film carrier are both supported by a single member permitting both the X-ray source and film carrier to orbit the patient at an uniform rate of travel. Means are also disclosed for reorienting the patient after completion of one-half of the excursion cycle in order to relocate the center of the axis of rotation with respect to the patient's head prior to X-raying the other one-half of the dental arch in order to provide the discontinuous, or split radiographic images.
In U.S. Pat. No. 3,045,118, apparatus is disclosed which automatically shifts the patient in order that the line of sight between the X-ray source and film bypasses the patient's spinal column and permits X-raying of the other half of the dental arch. Apparatus is also disclosed therein for continuously moving an X-ray source and extraoral film holder about the patient.
In U.S. Pat. No. 3,636,349, assigned to the present assignee, structure is disclosed for revolving the X-ray source and film carrier about the head of a patient who remains fixed in position while the center-line of the orbit continuously moves through an arcuate path approximating the arch of the patient's teeth. The patent further discloses film carrier means which may be used advantageously in the practice of the present invention.
Thus, the prior art discloses various types of structures, apparatus and mechanism for orbiting the X-ray source-X-ray film (tubehead-camera) assemblies in circular or arcuate paths; for varying film travel speed in accordance with tubehead-camera assembly movements; for shifting the patient in a chair; and for providing continuous or discontinuous type radiographic images.
In panoramic radiography, a selected layer of the dental arch structure is in sharp focus, called the focal trough, which is in a vertical curved plane having a horizontal dimension or width designed to portray radiographically part of the bone of the maxilla, part of the bone of the mandible, and the teeth. Present day continuous panoramic systems each suffer from an inherent infirmity of providing a radiograph having an extremely narrow focal trough width in the central region (incisors). This narrow focal trough width in the centrals is a serious limitation since it is virtually impossible to display both the incisal edge of the central teeth as well as their root areas, resulting in less available diagnostic information.
The present invention is capable of providing two simultaneous displays of two different focal trough areas at the centrals, or any different areas of the focal trough by means of a camera assembly containing two films which travel at different speeds past the camera slot and wherein only one exposure of the patient is required, markedly reducing the radiation dosage required.